NONPYROPHORIC METAL POWDER OF A METAL FROM THE GROUP IVb, Vb AND VIb OR THE ACTINIUM SERIES OF THE PERIODIC TABLE

ABSTRACT

A nonpyrophoric metal powder of Group IVb, Group Vb, Group VIb or the actinium series of the Periodic Table, having a particle size of 0.03 to 0.1 micron and a low surface area to volume ratio.

United States Patent METAL FROM THE cnoup IVB, vn AND vm on THE ACTINIUM SERIES OF THE rmuomc TABLE 1 Claim, 1 Drawing Fig.

Diverging We fer Cooled Nozzle Hydrogen Supply Wafer-Cooled Cathode Cooled Anode Plasma Jet Pain? of Applicafion 8 of Moral Halide [52] US. Cl 75/0.5 [51] Int. Cl B22! 9/00 [50] Field of Search 75/05 [56] Reierences Cited UNITED STATES PATENTS 3,049,421 8/1962 Allen et a1. 75/0.5 3,151,971 10/1964 Clough 75/0.5 3,165,396 1/1965 Goon 75/0.5

Primary Examiner-L. Dewayne Rutledge Assistant Examiner-W. W. Stallard Attorneys-Harry Goldsmith, Joseph G. Kolodny and Bryant W. Brennan ABSTRACT: A nonpyrophoric metal powder of Group lVb, Group Vb, Group Vlb or the actinium series of the Periodic Table, having a particle size of 0.03 to 0.1 micron and a low surface area to volume ratio.

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Attorney NONPYROPI-IORIC METAL POWDER OF A METAL FROM THE GROUP IVB, VB AND VIB OR THE ACTINIUM SERIES OF THE PERIODIC TABLE This application is a division of application Ser. 555,904, filed 7 June 1966, now US. Pat. No. 3,480,426.

This application is a division of application Ser. 555,904, filed 7 June 1966, now US. Pat. No. 3,480,426.

In gas-discharge physics the term plasma is used with reference to a partially or wholly ionized gas. lf the plasma as a whole has a directional velocity, it is called a plasma flow or plasma jet. Such a plasma jet can be produced, for example, by blowing a gas through an electric arc. In this manner temperatures of 20,000 C. and even higher can be attained. The velocity may range from a few meters per second to a multiple of the speed of sound.

lt is known that chemical reactions may be carried out in a plasma jet. In this way thermal decompositions, reductions with carbon or hydrogen, and halogenations have been performed; furthermore, a variety of nitrogen compounds has been prepared (see inter alia The Plasma Jet, Scientific American 197 [1957] No. 2,pp.80 et seq. and Industrial and Engineering Chemistry," volume 55, [1963] pages 16 to seq.)

It is further known that the gas stream may consist of an inert gas or of a reactive gas. For example when argon is used, a plasma jet is obtained which serves only as a source of heat; when, on the other hand, nitrogen or oxygen is used, the resulting gas is not only very hot but can, under suitable conditions, also be used for chemical reactions. When a graphite anode is used, reactions with carbon may be carried out in the plasma jet.

The present invention provides a process for the manufacture of finely dispersed, nonpyrophoric metals of the groups lVb, Vb, Vlb or of the actinium series of the Periodic Table wherein a halide of one of the said metals is treated with a hydrogen plasma, using for every molecular proportion of metal halide only to molecular proportions of hydrogen.

As metals of the groups mentioned above, designated as defined in the Handbook of Chemistry and Physics of Ch.D. Hodgeman, 1960, page 444, there are suitable titanium, ziroonium, hafnium, vanadium, niobium, tantalum, molybdenum, tungsten, uranium and thorium.

It is advantageous to use as metal halide those which are easiest to volatilize without decomposing. As a rule, this is true of the most highly halogenated metal halides. Preferred use is made of the chlorides, especially of TiCl,, ZrClj, NfCl.,, VCl,,, NbCl TaCl MoCl WCI WCI ThC l, and UCl,,. Instead of the molybdenum and tungsten chlorides there may be used with equally good results the oxychlorides MoCl. and WOCl, respectively.

In the case of a metal chloride wherein Me represents a tetravalent, pentavalent or hexavalent metal of the group mentioned above, the following reaction equations apply: MeCl,,+ 2 H f Me +4 l-ICl or 2 MeCl +5 H 2 Me+ 10 HCl or MoCl 3 H, :t Me 6 HCl.

in the case of an oxychloride the following reaction equation applies:

MeOCl, 3 H fl: Me 4 HC] H O Depending on its degree of dissociation, hydrogen enters the reaction partially in its atomic state.

Normally, the reaction in the plasma flame gives rise to metal in the pyrophoric form but, as has been observed, this is not the case when the reaction is performed with a relatively minor hydrogen excess, that is to say at a sufficiently high concentration of metal chloride in the plasma flame, because in this way metals of a less finely dispersed form are obtained. When the refractory metals of groups lVb, Vb, Vlb and of the actinium series are used at the ratio specified for the present process of 5 to 10 mols of hydrogen for every mo] of metal chloride, the average particle size of the resulting metal ranges from 0.03 to 0.1 p. In this context the term average particle size" is meant to indicate the so-called half-value particle size which is defined such that 50 percent of the particles of the whole collective are below this size. In view of general experience it must be said that it is indeed surprising that metals of the indicated range of particle sizes are nonpyrophoric. Using the definition in Staub" 22 [1962] in page 495, the term pyrophority is here used to describe the spontaneous ignition occurring in the absence of an extraneous igniter of a small quantity of a powder in the solid state on contact with air at room temperature. The nonpyrophoric character is also attributable to the shape of the particles. As has been revealed by electron microscopic examination, the present process furnishes predominantly particles having approximately the shape of cubes, octaheders or spheres. Thus, at the high reaction temperature, which is above the melting point of the metal formed, the resulting particles are not strongly fissured or porous, as is the case when the reaction is carried out at a low temperature. Accordingly, taking into consideration its particle size, the metal powder has a minimal surface and this has been verified by the surface areas measured and computed from grain-size-distribution graphs. In addition, it is known that the pyrophoric character of a substance also depends on fault arrangements of the lattice which constitute an increased energy state. The high reaction temperature used in the process of this invention is extremely favorable in this respect also because such lattice faults can heal much more quickly 3 than at a low temperature.

Another object of this invention is to provide improved metal powders by the present process. They are characterized by an average particle size from 0.03 to 0.1a, by a form factor F of 1.0 to L5 and an oxygen content not exceeding 3mg./square meter of surface. The definition of the average particle size has been given above. The form factor is defined as the ratio between the true surface of the particles (in actual practice measured according to a certain method) and the surface calculated from an assumed spherical shape of the particles; see W. Batel Korngroessenmesstechnik," Editors Springer, 1960. page 14. The form factor was in the present case determined as follows: Some 1,000 particles were measured and counted on electron microphotographs to enable the particle size distribution graph to be plotted as a first step. AS the characteristic length of a particle the diameter of a circle whose projection had the identical area was chosen. Using as a basis, spheres having these diameters, the surface of the particle collective can then be calculated from the distribution graph. The form factor as defined above is then obtained from this value and from the value resulting from the BET-measurement.

The use of metals having an average particle size below la is of special importance to powder-metallurgical processes, either as matrix metal in dispersion consolidation, or for the manufacture of alloys whose constituents have widely different melting points, or for sintering operations at lower temperatures. Fine refractory metals are also of importance in reactor technology and to catalysis.

The nonpyrophoric character of the metals obtained is very advantageous to their handling and further processing.

The present process is also distinguished by high yields which, as a rule, are better than percent.

In a further stage of the present process the resulting, very finely pulverulent and very voluminous metal is subjected to an aftertreatment to reduce its volume and to free it from contaminants (absorbed hydrochloric acid and low-valency halides). In this aftertreatment the powder is first rotated for several hours, whereby its bulk volume is reduced to one fifth and then under a vacuum from 10" to l0" mm. Hg. pressure calcined at a temperature at which the particles do not yet grow, preferably at a temperature from 600 to 800 C; if desired, the aftertreatment may be performed first in the presence of hydrogen and then under a vacuum. Unexpectedly, such an aftertreatment still leaves the powder nonpyrophoric. Oxidation in air proceeds only slowly and this is another feature facilitating the handling of the fine material.

The present process is generally performed by heating the metal halide to a temperature at which the vapor pressure of the halide is from one-half to 1 atmosphere, and if desired a carrier gas (argon or hydrogen) is conveyed over the surface of the halide. The resulting gas mixture is then injected into the plasmajet.

Depending on the conditions chosen, the reaction time in conditions:

Current intensity 200 ampercs are voltage 120 volts 74 standard liters the plasma jet is from 10 to 10 seconds and the tempera- 5 ture ranges from 2,000 m 5,000 c. C

e Plasma produced the 31d of a h' 'h At the exit opening of the diverging nozzle the plasmajet has a electric arc in a so-called plasma generator which is admean velocity of about 180 mJsecond and an average vantageously of the known design and comprises a wa rz re of about 3,20 C. At 1 cm. past the exit opening of cooled hollow copper anode and a cooled tungsten cathode. l the diverging nozzle 100g of gaseous Nbcla (with argon as To facilitate the mixing of the above-mentioned, relat vely carrier gas) per minute are fed into the hydrogen FL The lafge ameuht 9 halides the hydfogeh Plasmalet, thelet reaction mixture forms a brilliantjet of about cm. length. wldehed a dlvetglhg hezzle followlhg P (downstream of) Per minute 32g. of niobium, corresponding to a yield of 93 the burner. Then only is the hydrogen jet combined with the percent are obtained chloride .ie By wtdehihg the Plasmajet, good mixing with the 15 Five hundred grams each of the voluminous niobium metal halide and, as a result, a complete reaction is achieved powder accumulming in the reactor are d ifl d by being within a Short residence timeletting the mixing of the rotated at 9,000 revolutions per hour for l0 hours on rollers. reactants take Place y from tendency to form The material is then calcined for 6 hours in a weak current of of the apparatus, agglomeratiohs of the metal formed H (10 liters per hour) and then for another 4 hours under the apparatus and above all on the burner can be counvacuum at 300 C and thereupon 1 teraeteti Such agglomeratiohs would p y g the burner, The resulting, nonpyrophoric niobium contains 1.4 percent Specially when g Concentrations are used, 50 that the of oxygen. On exposure to air it undergoes oxidation and its process could not be performed continuously. it is another adi h increases l l b no spontaneousi i i occurs, vantage of this performance of the reaction that the large 8 specific Surface m re by h B T m d wa quantities reacted inside the flame do not impair the stability f n to be m- /gr m- The particle ize i ribution was ofthe electric arc. determined by counting about 1,000 particles on electron The invention is further describ d i th foll i micrographs with the use of a semiautomatic instrument; the reference being made to the drawing, the sole FIGURE of following distribution was foundi which is a diagrammatic section ofa plasma-jet generator seen 5 Percent below (W09 from the side. 25 percent below 0.0] 8 p.

in the drawing 1 is the supply of hydrogen which, as a rule, 50 Percent below P- flows in at right angles to the axis of the plasma jet at a rate 75 Percent below I which can be varied within wide limits; the water-cooled 95 Percent below 010 cathode 2 is adjustable relative to the cooled anode 3, while 4 that is to the halt-Value Particle Size was I represents th plasma jgt d d Th di i wate; The form factor F, calculated as described above, was 1.1. cooled nozzle 5 opens into the reactor 6, while the waste gas Tantalum, molybdenum, tuhgstehi Zirconium and hafnium duct 7 leads through settling vessels to remove as much dust as were Produced in a Similar y- The results of these expert" possible. The point at which the metal halide is supplied is ments are shown in the following represented at 8.

Reac- Particle size (share in t) tion Cnlcin. Spec Fol-111 condi- Throughput; per Yield temp. Oxygen surface 5% 25% 75% 05% [actor Metal tions minute percent 0.) percent (mfl/g.) I

120 g. T0015. 00 300 0.8 2.8 0. 020 0. 04 0.00 0.10 0. 10 1.4 00 700 0.0 0. 0 0.018 0. 03 0. 04 0. 00 0.00 1. 2 04 700 0. 7 0.1 0.012 0.02 0. 03 0. 04 0.07 1.3 05 800 1.8 13.0 0. 000 0. 01s 0. 03 0. 045 0.00 1.4 35 g. 111011.. 70 300 1. .s 0. 5 0. 000 0. 01s 0. 03 0. 045 0. 00 1.4

Reaction Conditions:

A=200 amperes, 1'20 volts, 74 standard liters of II: per minute. B=115 nmpcrcs, J8 volts, 24 standard liters of H2 per minute.

The metal halide is advantageously injected into the plasma jet through a supply tube made from quartz. As a rule, the metal is formed in the plasmajet under atmospheric pressure, but if desired a vacuum may be used. The points at which the metal halide is injected into the plasmajet must be determined in each case by suitable preliminary experiments.

EXAMPLE Manufacture of finely dispersed niobium The plasma generator was operated under the following 

